Considerable research has been conducted in the past to devise inorganic glasses exhibiting low transition temperatures (Tg), thereby enabling melting and forming operations to be carried out at low temperatures with consequent savings in energy costs. As conventionally defined, the transition temperature of a glass is that temperature at which notable increase in thermal expansion coefficient is recorded, accompanied by a change in specific heat (C.sub.p). More recently, it has been recognized that glasses demonstrating low transition temperatures are potentially useful materials for a host of applications including low temperature sealing glasses and glass-organic polymer composites. A very recent development disclosed in U.S. Pat. No. 5,043,369 (Bahn et al.) involves the production of glass-organic polymer alloys. Those alloys are prepared from a glass and a thermoplastic or thermosetting polymer having compatible working temperatures. Thus, the glass and the polymer are combined at the working temperature to form an intimate mixture; that is, the glass and polymer are in a sufficiently fluid state to be co-formed together to form a body displaying an essentially uniform, fine-grained microstructure in which, desirably, there is at least partial miscibility and/or a reaction between the glass and the polymer to promote adhesion and bonding therebetween. An article is shaped from the blend and then cooled to room temperature. Such articles exhibit chemical and physical properties comprising a complementary blend of those demonstrated by the particular glass and polymer. For example, the alloys frequently display a combination of high surface hardness, high stiffness, and high toughness. Alloys are distinguished from glass/organic polymer composites in that there is at least partial miscibility and/or a reaction between the glass and polymer which condition(s) is absent in glass/polymer composites.
Glasses having base compositions within the general zinc phosphate system have been found to be especially suitable for the glass component of glass-polymer alloys. Two illustrations of recent research in that composition system are reported below.
U.S. Pat. No. 4,940,677 (Beall et al.) discloses glasses exhibiting transition temperatures below 450.degree. C., preferably below 350.degree. C., consisting essentially, in mole percent, of at least 65% total of 23-55% ZnO, 28-40% P.sub.2 O.sub.5, and 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions selected from the group of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, and up to 35% total of optional constituents in the indicated proportions selected from the group of 0-6% Al.sub.2 O.sub.3, 0-8% B.sub.2 O.sub.3, 0-8% Al.sub.2 O.sub.3 +B.sub.2 O.sub.3, 0-15% Cu.sub.2 O, 0-5% F, 0-35% PbO, 0-35% SnO, 0-35% PbO+SnO, 0-5% ZrO.sub.2, 0-4% SiO.sub.2 and 0-15% MgO+ CaO+SrO+BaO+MnO, consisting of 0-10% MgO, 0-10% CaO, 0-10% SrO, 0-12% BaO, and 0-10% MnO.
U.S. Pat. No. 5,071,795 (Beall et al.) illustrates glasses exhibiting transition temperatures no higher than 350.degree. C. consisting essentially, in mole percent, of about 0-25% Li.sub.2 O, 25-50% ZnO, 5-20% Na.sub.2 O, 0-3% Al.sub.2 O.sub.3 0-12% K.sub.2 O, 25-37% P.sub.2 O.sub.5 0-10% SrO, with the amount of Li.sub.2 O+Na.sub.2 O+K.sub.2 O ranging from 15-35%. In addition, the composition may include 0.5-8% Cl and 0-5% F, as analyzed in weight percent, and up to 10% Cu.sub.2 O, up to 3% SiO.sub.2, and up to 8% total of at least one alkaline earth metal oxide may be included.
The above-described zinc phosphate glasses demonstrate relatively excellent resistance to chemical attack when compared to other phosphate-based glasses. Nevertheless, the search has been continuous to discover new glass compositions manifesting low transition temperatures with even greater chemical durability. Not only would these lower temperature glass compositions result in lower energy costs attributed the glass formation, they would also result in lower costs attributed to formation of glass/polymer alloys and composites. In addition, lower temperature durable glasses would also increase the number of compatible polymers available which could be co-processed with the glass to form glass/polymer composites and thermally co-deformed with the glass to form glass/polymer alloys. These factors, lowered formation cost and increase in potential polymers choices, would, in turn, likely increase the number of potential commercial applications.